Modeling quantum change in adolescent sexual initiation and condom use) A better understanding of early sexual initiation and condom use among adolescents is of great significance for prevention of HIV transmission. According to the dual-process systems theory, human behaviors are determined largely by two decision making systems. System 1 is a cognitive mechanism frequently used for routine or quick decisions (e.g., in emergency or under pressure); System 2 is a cognitive mechanism less frequently used for rule-based decisions by carefully weighing pros and cons of alternatives. According to this theory, adolescents' decisions regarding sex and condom use may contain both quick processes executed by System 1 and a gradual processes executed by System 2. Analytically, behavioral changes operated by System 1 will be seen as discrete because these changes are rapid, giving the appearance of having been executed without thinking. Behavioral changes executed by System 2 will be seen as continuous because critical thinking and careful assessment take time. Behavioral theories and models guiding much of our past research have focused on continuous changes, leaving discrete changes largely untouched. The co-existence of continuous and discrete processes in human behavior has been termed quantum behavior change (QBC) in the literature given their similarity to the well-known dual characteristics of waves and particles of light ray in physics. HIV behavior research may be advanced through the application of the QBC approach. The investigation of the QBC nature of a behavior requires methodologies capable of simultaneously modeling both continuous and discrete process; to date, such methods have not been well established. Cusp catastrophe modeling represents a promising approach, but several limitations of the estimation method prevent its use in HIV behavior research. Guided by the dual-process systems theory, in this project we will (1) extend and improve the estimation methods for cusp catastrophe modeling; (2) employ the improved methodology to simulate different risk and intervention scenarios to gain new insight into the complex dynamics of adolescents' sexual initiation and condom use behavior; (3) employ the same methodology to analyze the National Longitudinal Survey of Youth 1997 data to investigate factors associated with risky and safer sex, and data from two NIH-funded randomized trials we conducted to re-assess program effect; and (4) develop statistical software supporting the use of cusp catastrophe modeling method, making it available without charge to researchers. A team consisting of behavioral scientists and methodologists is assembled and well positioned to carry out the proposed research. To the best of our knowledge, this project is the first to address the QBC nature of adolescent sexual risk behavior using the cusp catastrophe modeling method and guided by the dual- process systems theory. Expected findings of this project will (1) add new data furthering our understanding of adolescent HIV risk behaviors and supporting more effective interventions for risk reduction and (2) provide new paradigms and analytical tools for researchers to advance their research agenda.